a) Field of the Invention
The present invention relates to a liquid-immersion type apochromatic objective lens system for microscopes which has a large numerical aperture (NA) and is capable of favorably correcting aberrations irrespective of variations in thickness of transparent cover plane parallel plates (cover glass plates) to be disposed selectively on the object side of the objective lens system.
b) Description of the Prior Art
In the field of the recent microscopy, the confocal laser scanning microscopy (CLSM), the digital deconvolution technique, etc. have made it possible to perform three-dimensional observations of biological cells. Since culture liquids used for enclosing the biological cells have refractive indices which are close to that of water (n.sub.d =1.33) and far from that of the ordinary immersion oil for objective lens systems for microscopes (n.sub.d =1.515), the microscopes can form images with high qualities of objects disposed at design focusing locations of the microscopes {normally lower surfaces of the cover glass plates (in case of microscopes which are configured for use with objective lens systems positioned underside)}, but imaging performance of the microscopes is abruptly degraded as object points are deviated from the focusing locations in a direction along optical axes.
Two methods have hitherto been contrived to correct this defect: one is to adopt a water-immersion type objective lens system which is designed to use water in place of the immersion oil such as an objective lens system for microscopes disclosed by Japanese Patent Kokai Publication No. Hei 5-196,875; and the other is to adopt an objective lens system which is equipped with a lens moving mechanism functioning to correct aberrations produced due to changes in refractive index of media used for enclosing cells such as an objective lens system for microscopes disclosed by Japanese Patent Kokai Publication No. Sho 61-162,015.
The objective lens system for microscopes disclosed by the abovementioned Japanese Patent Kokai Publication No. Hei 5-196,875 is suited for three-dimensional observations of biological cells, but this objective lens system is incapable of correcting variations of aberrations which are caused by changes in thickness of cover glass plates. Generally speaking, objective lens systems for microscopes which are designed for use with immersion oils exhibit optical performance scarecely degraded by the variations in thickness of the cover glass plates (within a range from 0.15 mm to 0.2 mm) owing to a fact that the immersion oils have refractive indices close to those of the cover glass plates. In case of the water-immersion type objective lens systems for microscopes which use water having a refractive index largely different from those of the cover glass plates, however, a slight change of 0.005 mm in thickness of the cover glass plates produces an influence on the optical performance of the objective lens systems or remarkably degrades optical performance, in particular, of objective lens systems which have large numerical apertures. Further, when biological cells have refractive indices which are different from that of water, the variations in thickness of the cover glass plates cause remarkable variations of aberrations, thereby making it hard to correct aberrations.
Furthermore, there are known, in the field of dry type objective lens systems for microscopes, a large number of objective lens systems which are equipped with means for correcting aberrations aggravated by changes in thickness of the cover glass plates.
An objective lens system for microscopes disclosed by Japanese Patent Kokai Publication No. Sho 56-142,508, for example, is a lens system which consists of three lens units, i.e., a first lens component configured as a cemented meniscus lens component having a positive refractive power, a second lens component having a positive refractive power and a third lens component having a positive refractive power; and is configured so as to correct variations of aberrations caused due to changes in thickness of cover glass plates by moving a small number of lens components or the second lens unit in a direction along an optical axis.
Moreover, an objective lens system for microscopes disclosed by Japanese Patent Kokai Publication No. Hei 3-50,517 is a lens system which consists of three lens components, i.e., a first lens component which comprises a positive meniscus lens element and has a positive refractive power, a second lens component having a positive refractive power, and a third lens component having a negative refractive power; and is designed so as to correct variations of aberrations caused due to changes in thickness of cover glass plates by moving the second lens component in a direction along an optical axis though the lens system has a relatively large numerical aperture of 0.8. Similarly, each of the objective lens systems for microscopes disclosed by Japanese Patents Kokoku Publication No. Hei 3-58,492, Kokoku Publication No. Hei 4-26,445, etc. is a lens system which is equipped with a means for correcting aberrations produced due to changes in thickness of cover glass plates by moving a specific lens component out of lens components used for composing the objective lens system in a direction along an optical axis.
However, these objective lens systems for microscopes are dry type lens systems having numerical apertures which are not so large as those of the liquid-immersion type objective lens systems for microscopes and cannot correct aberrations sufficiently favorably.
In addition, a liquid-immersion type objective lens system for microscopes which is compatible with various type of immersion liquids is disclosed by the above-mentioned Japanese Patent Kokai Publication No. Sho 61-162,015. This objective lens system consists of a front lens unit and a rear lens unit, and is configured so as to correct aberrations produced due to variations in refractive index of the immersion liquids by moving the rear lens unit and perform focusing by moving the objective lens system as a whole. However, this conventional example is incapable of correcting aberrations when it has a large numerical aperture exceeding 1.